In process automation technology, field measuring devices, especially two and four wire devices, are used for producing analog or digital measurement signals or measurement values representing parameters to be measured. Examples of the process parameters to be registered are mass flow rate, fill level, pressure, temperature, these being registered by means of a corresponding sensor, or a control variable, for instance an instantaneous valve position or an instantaneous rotation angle. See WO-A 98/44 317, WO-A 98/20 469, WO-A 98/14 850, WO-A 97/94 017, WO-A 97/07 444, WO-A 96/41 135, WO-A 96/05 484, EP-A 1 207 373, EP-A 1 108 984, EP-A 928 974, EP-A 780 665, U.S. Pat. No. 6,568,279, U.S. Pat. No. 6,330,517, U.S. Pat. No. 5,876,122, U.S. Pat. No. 5,829,876, U.S. Pat. No. 5,825,664, U.S. Pat. No. 5,812,428, U.S. Pat. No. 5,764,928, U.S. Pat. No. 5,764,891, U.S. Pat. No. 5,754,596, U.S. Pat. No. 5,573,032, U.S. Pat. No. 5,639,970, U.S. Pat. No. 5,495,769, U.S. Pat. No. 5,485,400, U.S. Pat. No. 5,481,200, U.S. Pat. No. 5,253,511, U.S. Pat. No. 4,926,340 for examples of such field devices having field device electronics for producing measurement signals representing process parameters.
Field measurement devices of the described type usually have a sensor for registering at least one process parameter and a transmitter connected, at least during operation, with the sensor for producing corresponding measured values for the registered process parameters. The measured values can then be sent, over a data transmission system connected electrically with the transmitter in an appropriate manner, to a superordinated process control system. By means of a process control computer provided in the process control system, the transmitted measured values are processed further and output in a suitable manner as measurement results, e.g. visualized on monitors, and/or converted into control signals for process actuators, such as magnetic valves, electro-motors, etc.
An elementary component of every data transmission system is an appropriate field bus, which is electrically connected with the involved field devices via appropriate bus connection cables. Suitable field bus systems are, by way of example, PROFIBUS-PA, FOUNDATION FIELDBUS, CAN-BUS or the like, in combination with appropriate interfaces, such as e.g. standard interfaces RS 232 or RS 485.
For the disengageable electrical connecting of field devices with the field bus, numerous plug coupling systems have established themselves, for example multipolar plug coupling systems with the coupling plugs of type M 12×1 or also ⅞″ familiar to those skilled in the art. Plug coupling systems of the described type are disclosed, for example, in DE-A 100 20 191, DE-U 87 06 150, DE-U 87 06 148, DE-U 86 13 225 or DE-U 86 13 221 and include, most often, connected with a transmitter-side end of the bus connection cable, for example embodied as a plug part, a coupling member having two, or more, contact elements, each being lastingly, galvanically connected with a conductor wire of the bus connection cable. Additionally, such plug coupling systems include a transmitter-side, coupling member having at least two or more contact elements, which are lastingly connected with the transmitter.
The two coupling members are adapted for disengageable, mechanical connection with one another such that each of the contact elements of the one coupling member contacts its mating one of the contact elements of the other coupling member, in order that the conductor wires of the connected bus connection cable are each galvanically connected with one contact element of the transmitter-side coupling member. The contact elements of the transmitter-side coupling member, which, as disclosed also in U.S. Pat. 471 831 or U.S. Pat. 471 829, can be embodied as coupling sockets, are predominantly provided in the form of contact pins.
Accordingly, the contact elements of the coupling member connected with the bus connection cable are contact sockets, of which each is suited to receive one of the two contact pins of the transmitter-side coupling member.
Usually, such plug coupling systems additionally have appropriately designed means for preventing an accidental disengagement of the two connected coupling means, such as, for example, a screwed connection using a union nut; see, in this connection, DE-A 100 20 191 or DE-U 86 13 221. Additionally, such plug coupling systems are usually provided with means which assure prevention of a wrong connecting of the bus connection cable. For example, a groove extending axially in the plugging direction can be formed into one of the coupling members, with a corresponding ridge being formed on the other coupling member.
Besides the primary function, namely the production of measurement signals, modern field devices exhibit numerous other functionalities, which support an efficient and safe management of the process being observed. In this regard, to be counted among these other functions are the self-monitoring of the field device, the storage of measurements, the production of control signals for actuating elements, etc. Because of this high functionality of the field devices, process control functions can, to an increasing degree, be moved to the field level, and, as a result, process control systems are becoming correspondingly decentrally organized. Additionally, these extra functionalities include e.g. even the start-up of the field device and its connection to the data transmission system. These and, if need be, still more field device functions are, as, for example, in fact shown in the above-mentioned WO-A 98/44 317, WO-A 98/20 469, WO-A 98/14 850, WO-A 97/94 017, WO-A 97/07 444, WO-A 96/41 135, WO-A 96/05 484, EP-A 1 207 373, EP-A 1 108 984, EP-A 928 974, EP-A 780 665, U.S. Pat. No. 6,568,279, U.S. Pat. No. 6,330,517, U.S. Pat. No. 5,876,122, U.S. Pat. No. 5,829,876, U.S. Pat. No. 5,825,664, U.S. Pat. No. 5,812,428, U.S. Pat. No. 5,764,928, U.S. Pat. No. 5,764,891, U.S. Pat. No. 5,754,596, U.S. Pat. No. 5,573,032, U.S. Pat. No. 5,639,970, U.S. Pat. No. 5,495,769, U.S. Pat. No. 5,485,400, U.S. Pat. No. 5,481,200, U.S. Pat. No. 5,253,511, U.S. Pat. No. 4,926,340, realized by means of a field device electronics, including a microcomputer with accompanying data storage circuit and software implemented therein.
The software can be inserted before, or during, start-up of the field device into a permanent memory, e.g. a PROM, or a non-volatile, persistent memory, e.g. an EEPROM, of the microcomputer and, as required, loaded into a volatile memory, e.g. a RAM, for the operation of the field device. This permits reconfigurations of the functions implemented in the field device electronics to be accomplished for the most part by simple changes of the stored software. These changes can include e.g. changes of individual measuring device parameters and also the loading of complete evaluation programs. In the application of non-volatile memory elements for storing the software, one possibility for reconfiguring the field device electronics is, for example, to download the changed software from a mass storage, e.g. a diskette, a CD-ROM or a magnetic tape, and/or via a null-modem into a volatile memory of the signal processing unit, e.g. into RAM. Then, the software to be changed can, as described, for example, in EP-A 1 108 984, be replaced by the software located in the volatile memory.
Both the loading of the software and also its reconfigurations can, for example, be accomplished by means of a special programming device on site, connected by an appropriate device connection cable temporarily to the field device.
For the disengageable connecting of the programming device with the transmitter, besides the above-mentioned plug coupling system, either there is provided, inserted between transmitter and programming device, an additional plug coupling system, which includes a further, transmitter-side coupling member, which likewise can be brought together with a complementary coupling member electrically attached to a transmitter-side end of the device connection cable, or else, as usual especially in the case of two-wire devices, the device connection cable is clamped parallel to the bus connection cable, for example directly thereon.
Ever more frequently, field devices of the described type are embodied as two-wire devices, so that, both the power supply of the field device and the signal transmission to the field bus can occur over one and the same two-wire line. Beyond this, it is to be noted that even existing applications with, for example, four-wire devices, in which the energy supply of the field device and the signal transmission to the field bus occur over two, mutually-separated, two-wire lines, are increasingly to be replaced by two-wire devices.
Considering that the installation expense for field devices, especially that of the electrical cabling, can be quite significant (costs equalling the purchase price of field devices are, in no way, unusual here), it would be of advantage, if the already laid supply lines, in any case, however, the bus connection cable, could continue to be used and not have to be replaced by completely new lines.
In doing this, it has, however, been found that, when using the already laid, four, or more, core, bus connection cables for the new two-wire devices, disadvantages concerning electromagnetic compatibility or even voltage-separation can arise. Problems with the electromagnetic compatibility can e.g. arise, when the electrical connection between programming device and field device is to be made over the same transmitter-side coupling member as the electrical connection between field bus and field device. This can, especially, also be the case, when the transmitter of the field device, or the plug coupling system, as the case may be, are so constructed that the connecting of the programming device can only be accomplished after disconnecting the bus connecting line from the transmitter.
One possibility for improving the electromagnetic compatibility of such plug coupling systems is e.g. to build additionally directly into the plug coupling system and/or into the transmitter appropriate noise filters or voltage-separating circuits, but this would lead to an increased circuit expense and, consequently, to significantly increased costs.